Field of the Invention
The present invention relates to a method and apparatus for processing digital signals prior to recording. More particularly but not exclusively the digital signals represent television video signals.
A channel code which had a minimum separation of 11/2 bit cells and which also was capable of statistically controlling the low frequency components was described in our co-pending U.K. Application No. A-2,141,906. For each successive group of eight bits it is possible, theoretically, (at least on average) to identify just over 453 discrete combinations and this should make it possible to construct a blockcode with controlled low frequencies with a minimum transition separation of 11/2 bit cells.
An alternative proposal has been made which operates on a bit-by-bit basis. Consideration has been given to the realisation of this alternative proposal but there are a number of difficulties which stem from the combination of the necessary high speed, the look ahead and the modulo-three requirements of parts of the coder.